The present invention relates to a plastification cylinder for the processing of plastifiable material in injection molding machines, the plastification cylinder being divided into heating and/or cooling zones and including heating elements and temperature sensors which are located in mutually independent zones.
Plastifiable material in the sense of the invention includes thermoplastic molding materials and interlaced molding materials, such as duroplasts and elastomers.
Due to the demand for injection molded items of high quality, especially items having stable properties and dimensions, considerable demands are made on the control elements of such machines. An important problem which has not previously been solved is that the injection molding machines, after the start-up phase, do not run in a stable manner in the sense that only essentially negligible temperature variations will occur in the plastification and injection unit, i.e., in the plastified material. Temperature variations in the plastified material result in changes in viscosity. Such viscosity changes can to some extent be compensated for, but have produced considerable difficulties, which have not yet been solved to a sufficient extent.
The temperature variations, which are caused mainly by friction in the material to be plastified, can have the effect that the finished injection molded articles have different material properties. This problem has so far also not been satisfactorily solved. This is due to the constructions of known plastification and injection cylinders. Due to the very high pressures occurring during injection, it is necessary to correspondingly dimension the wall thickness of these plastification and injection cylinders. Such plastification and injection cylinders have heating bands and thermosensors on the outside coating, which are generally arranged so as to be divided into three zones. The full heating output is only needed during the start-up phase.
These known plastification and injection cylinders have, however, a high heat capacity, so that the temperatures reported by the heat sensors on the outside wall do not even approach agreement with the temperature of the plastified material therein. Added to this is the time lapse of the phases between the temperature changes occurring in the plastified material and the temperature changes indicated by the temperature sensors.
Adding to the problem is the fact that the temperature of the plastified synthetic material is also subject to temperature variations. During the plastification, the temperature rises because of friction and then falls somewhat during the injection of the plastified synthetic material. To this extent it is understandable that considerable time phase shifts between the temperatures registered on the outside walls of the plastification cylinders and the actual temperatures in the plastified material occur, so that, based on the large time constant, complicated algorithms had to be utilized in the temperature regulation, in order to get a hold on this type of regulation. However, problems arise, particularly then when new materials, critically dependent on temperature control, are being processed.
Thus, e.g., in the production of CD disks made of polycarbonates, it is necessary to execute the plastification and homogenization of the material in the shortest possible time near the upper limit of the processing temperature.
Based on the so far appearing deviation from the rule, it has been necessary to provide for a high safety distance to the disintegration temperature and to accept possible qualitative disadvantages.
The present object of the invention is to provide a heatable plastification cylinder which permits the processing of synthetic material as near as possible to the upper processing temperature and which produces a sufficiently exact temperature behavior.